A nuclear power plant includes a nuclear reactor for heating water to generate steam which is routed to a steam turbine. The steam turbine extracts energy from the steam to power an electrical generator which produces electrical power. The nuclear reactor is typically in the form of a boiling water reactor having nuclear fuel disposed in a reactor pressure vessel in which water is heated.
The water and steam are carried through various components and piping which are typically formed of stainless steel, with other materials such as iron based alloys and nickel based alloys being used for various components inside the reactor pressure vessel.
It has been found that these materials tend to undergo intergranular stress corrosion cracking depending on the chemistry of the material, the degree of sensitization, the presence of tensile stress, and the chemistry of the reactor water. By controlling one or more of these critical factors, it is possible to control the propensity of a material to undergo intergranular stress corrosion cracking.
However, it is conventionally known that intergranular stress corrosion cracking may be controlled or mitigated by controlling a single critical parameter called the electrochemical corrosion potential (ECP) of the material. Thus, considerable efforts have been made in the past decade to measure the electrochemical corrosion potential of the materials of interest during operation of the reactor. This measurement, however, is not a trivial task, because the electrochemical corrosion potential of the material varies depending on the location of the material in the reactor circuit.
As an example, a material in the reactor core region is likely to be more susceptible to radiation assisted stress corrosion cracking than the same material exposed to an out-of-core region. The increased susceptibility occurs because the material in the core region is exposed to the highly oxidizing species generated by the radiolysis of water by both gamma and neutron radiation under normal water chemistry conditions in addition to the effect of direct radiation assisted stress corrosion cracking. The oxidizing species increase the electrochemical corrosion potential of the material, which in turn increases its propensity to undergo intergranular stress corrosion cracking or radiation assisted stress corrosion cracking.
Thus, a suppression of the oxidizing species is desirable in controlling intergranular stress corrosion cracking. An effective method of suppressing the oxidizing species coming into contact with the material involves the injection of hydrogen into the reactor water via the feedwater system so that recombination of the oxidants with hydrogen occurs within the reactor circuit. The recombination results in an overall reduction in the oxidant concentration present in the reactor which in turn mitigates intergranular stress corrosion cracking of the materials if the oxidant concentration is suppressed to low levels.
This method is conventionally called hydrogen water chemistry and is widely practiced for mitigating intergranular stress corrosion cracking of materials in boiling water reactors. When hydrogen water chemistry is practiced in a boiling water reactor, the electrochemical corrosion potential of the stainless steel material typically decreases from a positive value generally in the range of 0.050 to 0.200 V (SHE) under normal water chemistry to a value less than -0.230 V (SHE), where SHE stands for the standard hydrogen electrode. There is considerable evidence that when the electrochemical corrosion potential is below -0.230 V (SHE), intergranular stress corrosion cracking of materials such as stainless steel can be mitigated, and the initiation of intergranular stress corrosion cracking can be largely prevented.
Thus, considerable efforts have been made to develop reliable electrochemical corrosion potential sensors to be used as reference electrodes for determining the electrochemical corrosion potential of operating surfaces. These sensors are being used in boiling water reactors worldwide, with a high degree of success, which has enabled the determination of the minimum feedwater hydrogen injection rate required to achieve electrochemical corrosion potentials of reactor internal surfaces and piping below the desired negative value, -0.230 mV (SHE).
However, the sensors typically have a limited lifetime, in that some have failed after only a few months of use, while most have shown evidence of successful operation for approximately six to nine months. Only a few sensors have shown successful operation over a period of one fuel cycle, e.g. eighteen months in a U.S. boiling water reactor.
Recent experience with boiling water reactors in the United States has shown that the two major modes of failure of the sensor have been cracking and corrosive attack in the ceramic-to-metal braze used at the sensor tip, and the dissolution of the sapphire insulating material used to electrically isolate the sensor tip from the metal conductor cable for platinum or stainless steel type sensors.
The electrochemical corrosion potential sensors may be mounted either directly in the reactor core region for directly monitoring electrochemical corrosion potential of in-core surfaces, or may be mounted outside the reactor core to monitor the electrochemical corrosion potential of out-of-core surfaces. However, the typical electrochemical corrosion potential sensor nevertheless experiences a severe operating environment in view of the high temperature of water, typically exceeding 288.degree. C., relatively high flow rates, e.g up to several meters per second (m/s) or more, and the effects of high nuclear radiation in the core region. This environment complicates the design of the sensor, since suitable materials are required for this hostile environment, preferably configured to provide a water-tight assembly for a beneficial useful lifetime.
As indicated above, experience with the typical platinum electrochemical corrosion potential sensor has uncovered shortcomings leading to premature failure before expiration of a typical fuel cycle. Accordingly, it is desired to improve the design of electrochemical corrosion potential sensors to increase their useful life, e.g. to at least one fuel cycle.